Microwave heating device

Abstract: A microwave heating device having a cavity for heating a load and utilizing a method for feeding microwaves into the cavity using a controllable microwave distributor which enables load-independent control of the microwave field in the cavity.

Description: The present invention relates to a method for feeding microwaves to a microwave heating device, and to a microwave heating device comprising a microwave source for generating microwaves, a cavity in which a load is placed during a heating procedure and means for feeding microwaves into the cavity via feed points. There currently exist various models of microwave heating devices (for example microwave ovens) where the microwaves, which are generated by a microwave source or magnetron, are supplied in various ways to the cavity of the microwave heating device via one or more feed points. US 4,336,434, for example, discloses such a microwave oven. The oven is provided with a waveguide lying outside the cavity and two feed points. Additionally, a stirrer or antenna is disposed within the waveguide. When the microwaves are supplied to the cavity, a microwave field is formed having energy maxima and minima (also known as peaks and nodes) at certain positions. This phenonomen is well known in the technical field and results in the load (for example a food item which is to be cooked) receiving an uneven heating result. However, by disposing the stirrer in the waveguide, the positions of these peaks and nodes can be shifted during the heating procedure. The principle of operation is that the stirrer is set into rotation, for example by letting an airflow pass through the waveguide or with an electric motor, which results in a more even microwave field being produced in the cavity. The above patent is silent as to how the stirrer should be driven aside from that it is accomplished with an airflow or an electric motor. Other examples of prior art ovens, aside from those referred to in US 4,336,434, include i.e. DE 3 029 035, FR 1 378 280 and US 4,140,888. US 5, 015, 813 discloses a microwave oven with a certain specific position of the microwave oscillating antenne with respect to the ends of the waveguide The prior art ovens are constructed to give the most even microwave field possible in the cavity, i.e. an even power distribution. However, such a distribution does not provide the desired even heat distribution in a load which requires an uneven power distribution to become uniformly warm, e.g. a spread out, non-homogeneous load. The object of the present invention is to provide a method for feeding microwave energy into a cavity, and a microwave heating device for use therefor, which do not exhibit the abovementioned limitations and which enable control of the microwave field depending on the load which is to be processed. In pursuit of these objects, the method is characterized in that the microwaves are distributed to the respective feed points by a coupling factor between the microwave distributor and each of the feed points which is varied by controlling microwave distributing means, whereby each feed point is made, during a time segment of the heating procedure, to feed an individually determinable amount of microwave energy; and the heating operation is carried out by controlling the microwave distribution means in accordance with parameter values specifying a desired heating procedure for the load. One way to provide the abovementioned control in the method is characterized in that the microwave distributor is controlled by predetermined parameter values which are stored in a memory, the parameter values being collected from a table in accordance with a time function which specifies the heating procedure. A further way to provide the abovementioned control in the method is characterized in that measurements are continuously or intermittently sensed during the heating procedure by sensing means; the measurements are processed in a processor to obtain the relevant parameter values for the current state of the load; and the microwave distributor means is controlled in accordance with the thus obtained parameter values. A microwave heating device in accordance with the invention is characterized by microwave distributor means for distributing the microwaves to the feed points by variation of a coupling factor between the microwave distributor and the feed points; and control means for controlling the microwave distributor means in accordance with parameter values specifying a desired heating procedure for the load in order to carry out the heating procedure. Preferred embodiments of the microwave heating device of the invention may display one or more of the following characteristics: that the microwave heating device comprises waveguide means arranged to connect the microwave distributor with each feed point to conduct microwave energy into the cavity, the microwave distributor comprising a microwave distributor housing and the microwave distributor means; that the feed device is entirely located outside the cavity; that the microwave distributor means comprises a microwave distribution member which is rotatably disposed in the microwave distributor housing, a drive mechanism being arranged to move the microwave distributor member in accordance with control signals from the control means. In this fashion, a microwave heating device is provided in which an individually controllable amount of microwave energy can be fed to the cavity from a plurality of feed points. As the feed device in its entirety is located outside the cavity, it may advantageously be combined with a grilling element located in the roof of the cavity. In the case where the microwave distributor member is rotatably disposed in the distributor housing, the control may include that the member is rotated with a variable speed during an individual revolution or alternatively the member is positioned with a stepping motor. By controlling the microwave field, its phase, the amplitudes of the peaks and nodes, and their position in the cavity may be varied over time, as desired, during a heating procedure. In certain cases it is desirable to obtain as even a distribution of microwave energy as possible, while in other cases it is desirable to direct more energy toward certain positions in the cavity. This is determined by a number of different factors such as, for example, the nature and size of the load which influences how much of the microwave energy supplied to the load is respectively constituted by directly transmitted microwaves, and by microwaves which have been reflected from the walls, roof and floor of the cavity. With the abovementioned method and microwave heating device of the invention, these factors can be taken into account. A number of embodiments of a microwave heating device in the form of a microwave oven will, by way of example, be described below in more detail with reference to the accompanying drawings, in which: Figs. 1, 2, 3, and 3a & 3b, respectively depict a first, second, third and fourth embodiment of a feed device which may be disposed in a microwave oven; Fig. 4 depicts a microwave oven provided with the feed device depicted in Fig. 1; Fig. 5 depicts a microwave oven provided with the feed device depicted in Fig.3; Figs. 6A-6D schematically depict two examples of different ways of arranging a feed system; and Figs. 7A-7C depict the principles behind additional ways to provide different types of feed system. A feed device is depicted in Fig. 1 which may advantageously be used in a microwave oven. The microwaves generated by a magnetron (not depicted) are supplied to the feed device at an entry port 1 which is connected to a microwave distributor 2. The microwave distributor in this example comprises a microwave distributor housing 3 and a microwave distributor means or member 4. The microwave distributor means in this example is driven in a rotating movement by a drive device (not depicted). The microwaves are subsequently conducted via waveguide 5 to the various feed points which are disposed in the walls of the cavity. Figure 1 depicts four waveguides, each of which is disposed at right angles to the adjacent waveguides. This is clearly not obligatory, rather the feed device can be provided with an arbitrary number of waveguides which may assume any desired angle relative to each other and the microwave distributor. When the microwaves have entered the microwave distributor housing 3, the position of the microwave distributor means determines how large a fraction of the total amount of microwave power will be distributed to the respective waveguides. This is effected through the coupling factor between the respective waveguides and microwave distributor housing, which may equally be expressed between the respective feed points and the microwave distributor housing, being varied as a function of time as the microwave distributor means rotates. The coupling factor may, for example, be expressed as a percentage of the total microwave power which is distributed to the respective waveguides or as a function of the phase difference resulting between the waveguides. This gives rise to the above described capability to control the microwave field in the cavity. The microwave field obtained in the cavity is determined, inter alia, by how the distributor means rotates as a function of time. The shape of the microwave distributer means or member may be executed in many different ways and is not limited to the depicted example. An alternative embodiment of the feeding device shown in Fig. 1 is depicted in Fig. 2. The upper portion of the microwave distributor 2 is depicted separated from the microwave distributor for the sake of clarity. Instead of arranging waveguides which extend from the sides of the microwave distributor housing, the microwave distributor housing is provided with exit ports 6 which are disposed directly in front of the feed points disposed in a wall of the oven cavity (see below). The microwave distributor in Fig. 2 fulfills the dual roles of distributing the microwaves and conducting them to the respective feed points. The waveguides in Fig. 1 are not required and their function can be said to have been integrated in the microwave distributor. Fig. 3 depicts an alternative embodiment of the feed device of the invention. Here, the microwave distributor means is constituted by a plate 7 which is secured to a pivotable axle 8. The microwaves are supplied to the microwave distributor housing via the entry port 9, from which they are subsequently distributed to the waveguides 10 and the feed points in the cavity. The plate can execute a movement between two end positions, so that the coupling factor between the respective waveguides and the microwave distributor housing may be varied in a corresponding fashion to that described above. This movement may, for example, be provided by a motor 11 whose drive axle constitutes the axle 8. Figs. 3a and 3b schematically depict a further development of the feed device of Fig. 3, respectively in a partial sectional view from the feeding side of the waveguide 10, and a partially sectional side view. The waveguide 10 is provided with a feed portion 19 on which a magnetron 12 is secured for the supply of microwaves. Located in the feed portion is a plate 7 which is pivotable on an axle 8, arranged in a fashion corresponding to Fig. 3, to distribute microwave power between upper and lower branches of the waveguide 10 which have respective exit ports 17, 18 intended to communicate with corresponding feed points in the cavity of a microwave oven. The breadth of the waveguide branches expands in the direction towards the outlet ports 17, 18. A vane 15 is located in the waveguide and is pivotable around an axle 16 disposed coplanar with, and transverse to, the axle 7. Rotating the vane 15 alters the effective breadth of the respective waveguide branch and thus their electrical length. Different electrical lengths give a corresonding phase difference between the microwaves which exit from exit ports 17, 18. This phase difference affects the microwave field distribution in the cavity which can thus be controlled by rotation of the vane, for example with a stepping motor in accordance with the control parameters. In this case, the microwave distributor is partially integrated in the waveguide 10 itself by constituting the vane 15 secured therein, the feed portion 19 and the plate 7. In accordance with the invention, the microwave distributor means is driven in a controlled fashion. This is brought about by selecting parameter values, which in turn determine the food preparation results for different types of food items. These parameter values may, for example, be preparation time, power as a function of time and how the microwave distributor means should be driven (i.e. the position of the microwave distribution means as a function of time). This drive information for the microwave distributor means may be specified as a time function and collected from a table stored in a memory. The drive information may be obtained empirically by a testing process where the manufacturer of the microwave oven picks out the optimal parameter values for different preparation situations, these being then stored in the memory. In this fashion, the desired microwave field is subsequently provided. Alternatively, the drive information may be generated during the heating procedure with sensors located in the cavity, the sensors measuring different physical quantities such as, for example, temperature and weight. These quantities are processed in a processor for conversion to suitable parameter values, which are then used to control the heating process as described above. In the case of a movable distributor means or member in the form of a rotatable plate as in Figs. 1 & 2, control may be provided by varying the speed of rotation during a revolution or by stepwise translating movements. In a corresponding fashion, the plate 7 and vane 15 may be pivoted with a variable speed or stepwise. A cavity to a microwave oven is schematically depicted in Fig. 4 and is provided with the feed device shown in Fig. 1. The magnetron 12 generates microwaves and these are then distributed in the described manner to the waveguides. The placement of the magnetron is clearly not limited to that shown in the figure, but can be disposed at a suitable place, as desired, a waveguide being disposed between the magnetron and the entry port 1 on the feed device. The exit ports of the waveguides are located over feed points 13 in the roof of the cavity so that the microwave feed in this case is effected from four different feed points. The microwave distributor means is driven by a drive device, schematically depicted here, such as an electric motor. The motor is controlled by the drive information which is stored in the memory, as is described above. If the microwave oven is not equipped with sensors, a user of the oven may, for example, input information via a button panel (not depicted) which selects the relevant parameter values for the current processing procedure. If the microwave oven is equipped with sensors, the measurements of physical quantities from the sensors is used as in-signals to the processor, the measured values being converted to relevant parameter values and supplied to the motor as drive information. Fig. 5 depicts a cavity to a microwave oven provided with the feed device shown in Fig. 3. Advantageously, this embodiment may be dimensioned so that it functions as a resonant feed system in accordance with SE 9003012-3 which has been additionally fitted with a microwave distributor, as described above. This gives an additional capability to control the microwave field in the cavity by integrating the present invention with this resonant feed system. In the figure, the feed system is depicted with a vertical placement on one of the side walls of the cavity. Clearly, the feed device can be located in any desired orientation on any of the walls of the cavity. Figs. 6A-6D depict two examples of how a complete feed system can be constructed. Fig. 6A depicts a feed device with three feed points. Feed points I 2 and I 3 , if so desired, may be arranged so that they provide resonant input as described above, and the feed point I 1 may be fitted with a stirrer. The microwave distributor D is controlled during the processing procedure to distribute the energy between these feed points, whereby it is possible to select, to varying degrees, between resonant feed through feed points I 2 and I 3 , and feed point I 1 . Fig. 6B is, in principle, a feed device in accordance with Fig. 3 where one feed point I 2 is disposed on one of the side walls of the cavity and the second feed point I 2 is disposed on the roof of the cavity, where a stirrer in front of the feed point can be arranged in a corresponding fashion as in Fig. 6A. Figures 6C and 6D are simplified explanatory sketches of Fig. 6A and 6B respectively. Finally, Figs. 7A-7C show three examples of how a microwave system may be configured. Fig. 7A shows a microwave distributor D 1 which has N individual waveguides which lead to N individual feed points I 1 ,...,I N . Figure 7B shows a configuration where N-1 individual microwave distributors D 1 ,...,D N-1 . effect feeding through N individual feed points. Figure 7C is a further example where feeding is effected through N individual feed points. The number of microwave distributors D 1 , D 2 ,... with this configuration is given by the formula: 1 < number of microwave distributors < N-1.

Claims: A method for feeding microwaves to a microwave heating device which comprises a microwave source (12) for generating microwaves and a cavity in which a load is placed during a heating procedure, the microwaves being fed via feed points (13) to the cavity; the microwaves are supplied to a microwave distributor (2) which is common to the feed points, and is located outside the cavity; the microwaves are conducted through the microwave distributor to the feed points; characterized in that the microwaves are distributed to the respective feed points by a coupling factor between the microwave distributor and each-one of the feed points which is varied by controlling microwave distributing means (4,7,15), whereby each feed point is made, during a time segment of the heating procedure, to feed an individually determinable amount of microwave energy; and the heating procedure is carried out by controlling the microwave distributor means in accordance with parameter values specifying a desired heating procedure for the load. A method according to claim 1, characterized in that the microwave distributor means is controlled by predetermined parameter values which are stored in a memory, the parameter values being collected from a table according to a time function which specifies the heating procedure. A method according to claim 1, the microwave oven comprising one or several sensors for taking measurements in the cavity, characterized in that the measurements are continuously or intermittently taken during the heating procedure by the sensor(s); the measurements are processed in a processor to obtain the relevant parameter values for the current condition of the load; and the microwave distributor means is controlled in accordance with the thus obtained parameter values. A microwave heating device comprising a microwave source (12) for generating microwaves, a cavity in which a load is placed during a heating procedure, and means for feeding microwaves into the cavity via feed points (13); a feed device incorporated in the means for feeding microwaves, which feed device comprises: an entry port (1) for receiving microwaves from the microwave source; a microwave distributor (2) which is common to the feed points and to which microwaves are conducted from the entry port; characterised by : microwave distributor means (4,7,15) for distributing microwaves to the feed points by variation in a coupling factor between the microwave distributor and the feed points; and by control means for controlling the microwave distributor means in accordance with parameter values specifying a desired heating procedure for the load in order to carry out the heating procedure. A microwave heating device according to claim 4, characterized in that it comprises waveguide means (10), arranged to connect the microwave distributor with each feed point to conduct microwave energy into the cavity, the microwave distributor comprising a microwave distributor housing (3) and the microwave distributor means. A microwave heating device according to claim 4 or 5, characterized in that the feed device is entirely located outside the cavity. A microwave heating device according to claim 5 or 6, the microwave distributor means is movably disposed in the microwave distributor housing; and a drive device (11) is arranged to move the microwave distributor means in accordance with a control signal from the control means. A microwave heating device according to claim 7, characterized in that the microwave distributor housing is cylinder shaped with the waveguide means connected to openings in the cylindrical surface of the cylinder; the microwave distributor means comprises a curved plate which is disposed in the microwave distributor housing and whose height and curve are adapted to the inside of the cylindrical surface; and the microwave distributor means is arranged to perform a, preferably rotating or reciprocating, motion while the curved plate successively passes and screens the openings. A microwave heating device according to claim 7, characterized in that the microwave distributor housing is cylinder shaped, exit ports (17,18) being located in the microwave distributor housing which are disposed directly facing feed points into the cavity; the microwave distributor means comprises a curved disc which is disposed in the microwave distributor housing and whose height and curve are adapted to the inside of the cylindrical surface; and the microwave distributor means is arranged to perform a, preferably rotating or reciprocating, movement while the curved disc successively passes and screens the ports. A microwave heating device according to claim 7, characterized in that the microwave distributor housing is integrated with the waveguide means; the microwave distributor means comprises a plate (7) which is disposed in the microwave distributor housing and whose length and breadth are adapted to the inside of the microwave distributor housing; and the microwave distributor means is arranged to perform a, preferably rotating or reciprocating, movement while the plate successively screens the waveguide means. A microwave heating device according to any one of claims 5 to 10, characterized in that the waveguide means incorporated therein comprise a movable vane (15) whose length is adapted to the length of the waveguide and whose breadth is adapted to the height of the waveguide, the vane being movable in the widthwise direction of the waveguide means to affect the electrical length of the waveguide means and the phase relation of the microwaves which are fed from an exit port in the waveguide means. A microwave heating device according to claim 11, characterized in that the waveguide means comprises a substantially straight waveguide having two substantially similarly shaped branches, each branch extending with increasing breadth from a centrally located microwave entry port to a microwave exit port (17,18) in communication with the feed points in the cavity; and the vane is pivotably disposed adjacent an edge side of the wave guide, pivoting around an axle at the middle of the vane which extends transversely to the vane. A microwave heating device according to any one of claims 7 to 10, characterized in that the drive device comprises an airflow generating means which drives the microwave distributor means. A microwave heating device according to any one of claims 7 to 10, characterized in that the drive device comprises a motor (11) for driving the microwave distributor means.

Inventors: EKSTROM ANDERS, JOHANSSON SVEN-OLOF, EKSTROM, ANDERS, JOHANSSON, SVEN-OLOF, EKSTROM, ANDERS, C/O WHIRLPOOL ITALIA S.R.L., JOHANSSON, SVEN-OLOF, C/O WHIRLPOOL ITALIA S.R.L.
Applicants: WHIRLPOOL EUROP, WHIRLPOOL EUROPE B.V.
Application year: 1994
Main classification: H05B6
IPC codes: H05B6